Heretofore, in thermal and/or acoustical insulation materials composed of an inorganic fiber such as glass wool or rock wool, phenol resin-based binders composed mainly of a phenol-formaldehyde resin (or a resol-type phenol resin) have been widely used as the binder that binds the fibers to each other. These phenol resin-based binders cure in a relatively short time upon heating to provide cured products having strengths. Therefore, thermal and/or acoustical insulation materials made of inorganic fiber using these are excellent in shape stability, thickness recovery when opened after compressed packaging, and resistance to bending, and the like.
However, when the phenol resin-based binder is used, formaldehyde is released in the air in the manufacturing process of the product, predominantly during curing of the binder. Accordingly, the treatment and/or handling of the released formaldehyde have been problematic. In recent times in particular, to attempt to decrease environmental loads, regulations by the law or the like have been necessitated controlling the amount of formaldehyde released in the air. Thus a thermal and/or acoustical insulation material made of inorganic fiber having less environmental loads is demanded and a number of proposals have been made.
For example, JP-A-06-184285 discloses a curable aqueous composition containing (a) a polyacid containing at least two carboxylate groups, acid anhydride groups, or salts thereof, (b) a polyol containing at least two hydroxyl groups, and (c) a phosphorus-containing accelerator, wherein a ratio of the equivalent of the carboxylate groups, acid anhydride groups, or salts to the equivalent of hydroxyl group is about 1/0.01 to about 1/3, and wherein about 35% or less of the carboxylate groups, acid anhydride groups, or salts thereof is neutralized with a nonvolatile base.
Further, U.S. Pat. No. 6,331,350 discloses a binder for glass fiber that includes a water-soluble composition containing a polycarboxylic acid polymer having a number average molecular weight of less than 5,000 and a polyol, wherein the pH of the binder is adjusted to less than 3.5.
Further, JP-A-2000-508000 discloses a formaldehyde-free aqueous binder that contains A) a polymer obtained by radical polymerization, the polymer containing 5 to 100 mass % of an ethylenically unsaturated acid anhydride or an ethylenically unsaturated dicarboxylic acid whose carboxylate groups can form an anhydrous group, and B) an alkanolamine having at least two hydroxyl groups.
Also, a number of binders for inorganic fibers that are composed mainly of the above-mentioned polycarboxylic acid have been proposed.
The above-mentioned polycarboxylate resin-based binder such as an acrylic resin-based binder has a property of low reactivity in a pH range of weakly acidic to weakly basic for the carboxyl group therein with the hydroxyl group in polyols used as crosslinking agents, therefore an esterification reaction is difficult to sufficiently proceed. As a result, the crosslinking reaction of the binder is hard to be completed. Thus, when the above-mentioned binder is adjusted to a pH ranging weakly acidic to weakly basic, various physical properties of the thermal and/or acoustical insulation material made of inorganic fiber tend to be deteriorated. For example, physical properties of shape recovery and/or resistance to bending tend to be deteriorated due to the deterioration of the binder under at high humidity. For this reason, the acrylic resin-based binder is not suited for use under a pH condition ranging weakly acidic to weakly basic. Usually, the acrylic resin-based binder is employed by adjusting the pH thereof to a strongly acidic pH raging about 3 to thereby promote the crosslinking reaction. However, in such a case, manufacturing equipments such as a binder feeding pipe, a spraying apparatus, a mesh conveyor for deposition of inorganic fibers tend to be corroded with the acids and hence there arises a problem that the maintenance of the apparatuses as well as the apparatuses themselves are costly. There also arises a problem that the disposal of strongly acidic waste discharged is costly.
Further, the crosslinking of the above-mentioned acrylic resin-based binder is created with ester bonds. In the case of inorganic fibers, glass fibers in particular, this causes a problem that the alkali metal component in the glass is eluted in a form of alkali ion due to the moisture in the air and then the alkali ion hydrolyzes ester bonds at the crosslinked portions of the binder, thus deteriorating the binder's binding force for the fibers.
Therefore, it is an object of the present invention to provide; an aqueous binder for inorganic fibers, which is formaldehyde-free, has excellent strength, and is suitable for use under conditions of pH ranging weakly acidic to weakly basic, and whereby a cured product composed of the binder having excellent strength even when used after a period of time is obtained; and a thermal and/or acoustical insulation material made of inorganic fiber using the same.